1. Field of the Invention
The present invention relates to the structure and drive technique of the cathode ray tube used as an image display device for television sets and computer displays.
2. Description of the Prior Art
According to a conventional image display device, there is proposed by the present inventor a flat plate type cathode ray tube described in Japanese Laid-open Patent Publication No. Hei 1-130453. FIG. 7 shows the internal electrode arrangement of the flat plate type cathode ray tube.
The cathode ray tube shown in FIG. 7 is composed of line-shaped cathodes 1-1, . . . , 1-4 as electron beam emission sources, a rear electrode 2 disposed on the opposite side to the image display screen 9 so as to face the cathodes 1, an electron beam take-out electrode 3, an electron beam modulation electrode 4, a vertical focus electrode 5, a horizontal focus electrode 6, horizontal deflecting electrodes 7, and 7a, and vertical deflecting electrodes 8 and 8a, and a display screen 9 with a fluorescent material applied thereon. These components are enclosed in a flat vacuum glass container (although not shown, a face plate 11 and a rear plate 12 constitute parts thereof).
Each line cathode 1 extends in the horizontal direction, and L pieces (L=4 in FIG. 7) of such line cathodes 1 are arranged vertically at a proper spacing. An electron beam taken out from the line cathode 1 is in a sheet-spread form, and is divided into M sections of fine electron beams by passing through holes of the electron take-out electrode 3. Then the fine electron beams are applied to the electron modulation electrode 4. The electron beam modulation electrode 4 is divided into M segments each extending in the horizontal direction so as to control the amount of electron beams to pass through the vertical segment independently and simultaneously (in FIG. 7, only 9 segments are shown).
The vertical focus electrode 5 and the horizontal focus electrode 6 respectively focus the beams in the vertical direction and horizontal direction.
The horizontally deflecting electrodes 7, 7a are provided to hold horizontally divided electron beams between two electrodes 7 and 7a, and deflect the beams horizontally by a potential difference given between the pair of electrodes 7 and 7a.
Similarly, the vertically deflecting electrodes 8, and 8a are provided to hold all the electron beams of one scanning line between a pair of electrodes 8 and 8a deflect the beams vertically by a potential difference given between the pair of electrodes.
Respective electron beams subjected to focus, modulation and deflections are accelerated by a high voltage applied on the screen 9 so as to effect bombardment of electron beams on the fluorescent film on the screen 9 for the generation of fluorescence. A fluorescent stripe is arranged so that as an example, one triplet set of R G B corresponds to each through hole in the electron beam modulation electrode 4.
Next, the method of giving deflection voltage in the conventional example will be described by showing waveforms in FIG. 8, taking the case of 480 scanning lines for the NTSC system as an example. The horizontal deflection is effected by step-shaped deflection signals h, and h.sub.1 shown in FIG. 8. Since one deflection width during one H period is equal to a scan distance over one triplet set of R G B, the deflection signals h and h.sub.1, which are synchronized with the horizonal synchronization signal H.D, are prepared in stepped waveforms that rise or fall at a rate of H/3 period. Accordingly, the electron beam stands still on respective fluorescent elements for each H/3 period.
On the other hand, the vertical deflection is effected by step-shaped deflection signals v and v.sub.1. The time period in which electron beam is taken out from each cathode is (240/L)H, as indicated in cathode drive pulses K1 to KL, and each beam is deflected vertically in (240/L) steps (in FIG. 8, L=80, so that the number of vertical deflection step is 240/80=3). In the entire screen, during one vertical scanning period (one field), 240 lines of rasters are made by vertical deflections of 240 steps. In the next field, the interlace scanning is effected by shifting the voltage value so that the electron beams lands between rasters made in the previous field.
The horizontal deflection and vertical deflection are effected as described above, and the modulation is effected by changing the modulation signal w to R, G, and B in accordance with the deflection so as to form an image display section 10 with three vertical and three horizontal light emitting spots excited by one electron beam. A display image of one screen is obtained by aligning the image display sections regularly on the screen.
However, in the above flat plate type cathode ray tube, the horizontal deflection electrodes 7 and 7a, and the vertical deflection electrodes 8 and 8a are adjacently confronting each other and are electrically coupled to each other by comparatively large capacitances generated therebetween (in a 6" size screen, about 1000 pF), which adversely affects the deflection signals. Furthermore, even in the case where the horizontal deflection electrodes and the vertical deflection electrodes are not adjacently disposed, a similar phenomenon may take place between the horizontal deflection electrodes and the other adjacent electrodes or between the vertical deflection electrodes and the other adjacent electrodes.
Taking the horizontal deflection electrodes and vertical deflection electrodes as an example, when the output impedance of the deflection circuit driving the horizontal deflection electrodes 7 and 7a, and that of the deflection circuit driving the vertical deflection electrodes 8 and 8a are represented by RH and RV, respectively, and the capacitances between the horizontal deflection electrode 7 and the vertical deflection electrodes 8 and 8a are represented by C.sub.7-8 and C.sub.7-8a, respectively, and those between the horizontal deflection electrode 7a and the vertical deflection electrodes 8 and 8a are represented by C.sub.7a-8 and C.sub.7a-8a, respectively, the equivalent circuit for these deflection electrodes is represented as shown in FIG. 9. Since C.sub.7-8 is not equal to C.sub.7a-8 and C.sub.7-8a is not equal to C.sub.7a-8a, the higher harmonics v.sub.h and v.sub.h1 of the horizontal deflection signals h and h1 (FIG. 10a) induced in the vertical deflection electrodes 8 and 8a become signals having opposite polarities and different wave heights (FIG. 10b), and the combined waveform v.sub.h +v.sub.h1 (FIG. 10c) is superimposed on the original vertical deflection waveform so as to cause the change in the beam landing or focusing state, resulting in the image distortion, such as the unevenness in color or brightness.